This invention relates to a circuit arrangement for reducing the residual voltage on partially disconnected loads, such as control coils of electromagnetic switchgears with long control lines, by connecting an additional load in parallel.
Residual voltages of this type are generated by resistive, capacitive, or inductance coupling. They appear particularly in the case of magnetic coils energized by AC current, such as the contactor's driving magnet, which must be actuated via long control lines. After the control switch disconnects, there occurs a capacitive leakage current which, in the case of a sufficiently high line capacitance, can be so high that the consequent residual voltage exceeds the bias of the magnet coil, so that a proper de-energizing of the magnet coil is prevented. This phenomenon may also occur when electronic output stages are used as control switch whose power semiconductors are wired up with RC-networks.
In the case of the familiar circuit arrangements of the type described above (Schmelcher, T., "Handbuch der Niederspannung", Berlin-Munich 1982, pp. 148, 149) the additional resistive or capacitive loads are connected in parallel to the magnetic coils whose current consumption causes a drop in the residual voltage below the bias value of the magnet when the control switch is disconnected. If additional resistive loads are being used, considerable power losses occur in the on-position. When additional capacitive loads are connected to the load, the undesirable phenomena of resonance may occur. Furthermore, if we are to obtain an adequate effect, high capacitance values are required which are both costly and space-consuming.